The present invention relates to formable radionuclide sources of beta or gamma radiation, and particularly the use of such sources in industrial or medical devices.
Encapsulated, usually described as “sealed”, radionuclide sources of both beta and gamma radiation are widely used in industry and medicine. Typical examples include: (a) sources for thickness gauging, for process control, for weld inspection; for radiation processing and curing, for static electricity elimination and for industrial and medical instrument development, evaluation and calibration; and (b) sources for human radiation therapy by interstitial, intra-cavitary, intra-vascular, and surface methods; for external surface or sub-surface irradiation by means of nuclide-bearing applicators; for teletherapy; and for radiation detector adjustment, efficiency determination and other calibration or dosimetric purposes.
The masses, and hence the volumes of radioactive material in many commonly used irradiation devices, are small. For example, the mass of a pure P32 sample whose activity is 1 mCi (millicurie) is less than 10−10 g (grams) and even in the extreme case of Ra226 whose atomic mass and half life are large, a 1 mCi sample has a mass of only about 1 mg (milligram). To manufacture sources of customary activities in physical dimensions appropriate to their proposed uses, it is therefore frequently necessary to distribute the active material throughout the volume of, or attach it to the surface of, a non-active matrix or filler of the desired size and shape; the resulting source/filler combination must then be encapsulated or sealed to prevent escape of radioactive material.
Exemplary encapsulation techniques include (1) use of a thin double walled metal container, such as a tube or needle containing the active material in the form of a powder or microbeads, mixed with an inert filler to confer sufficient bulk to fill the container; (2) evaporating a radioactive solution form the surface of the matrix; (3) enclosing an active powder (again accompanied by a volume-augmenting filler) in a thin-walled plastic or metal-foil envelope which is then affixed to a substrate already formed in the desired source shape; (4) impregnating, e.g., a plastic sheet with active material which is mechanically forced into the sheet by a process such as hot rolling; and (5) suspending an already active material in a liquid monomer which is later polymerized in the desired shape. These techniques are particularly difficult to apply when the desired shape is complex; furthermore, the activity distribution achieved may be unacceptably non-uniform and most important, the radiation hazard during manufacture is high.